The present invention relates to optical data storage systems, optical data storage members and is particularly applicable to data bases. The invention is also applicable to optical data recording systems as disclosed in PCT Application GB No. 82/00179.
Two entirely different optical data storage systems have found commercial acceptance, one employing microfilm and the other employing optical laser storage methods.
In microfilm systems, high capacity optical data storage is effected photographically on light-sensitive microfilms, such as microfiche. Capacities of up to 10 million bits/mm.sup.2 have been achieved, the maximum being limited by the quality of the photographic emulsions available. The production of microfilms for a very high capacity system is therefore very expensive. Moreover, some data bases (e.g. automobile part catalogues) require large numbers of identical copies, so that the expense of reproducing such microfilms has to be taken into account. On the other hand, a microfilm reader is relatively inexpensive as it does not rely on laser illumincation and can employ a relatively cheap optical imaging system. Nevertheless, it can be a time consuming operation to located desired information on microfilm, although this problem has been alleviated to some extent by the use of microfiche, which involves reproducing a large number of pages of data on a single sheet which can be searched manually in two dimensions.
U.S. Pat. No. 4,179,743 discloses a microfiche system having a two-dimensional array of `pages` arranged in a rectangular storage zone, the pages having distributed amongst them control data for page location. The controdata is an intrinsically ordered set of alphanumeric data containing classifying data pertaining to associated pages. Such a system can only be used with pre-ordered data, such as a dictionary in alphabetic order. Storage space is reduced by distributing the control data through the storage zone.
Optical laser storage systems using discs have been made wherein data is stored in the form of a coded series of pits in concentric rings, or spirals, on the surface of a transparent disc. The data is read using a source of coherent laser illumination focused on each of the pits in turn as the disc is rotated. The spot size of the illumination must be less than or comparable to the area of a single pit.
The light defracted by the pits, either on reflection or in transmission, is monitored to reconstruct electronically an image of the storage data. The data in this case is necessarily encoded and so is not directly readable, requiring the use of electronic decoders to convert the stored data into signals suitable for a video display unit. Such a system makes it possible to produce very good random-access times. One such system is described on pages 26 to 33 of the IEEE Spectrum of August 1979. In that system each set of information is accompanied by an address heading to locate information on the disc. This system for location is efficient in that context, where the data is digitally encoded and is read serially whilst the disc is spinning at high speed. Such a system, attaching a heading to each item of data, takes up space on the disc and would not clearly be efficient in microfilm-type applications.
A disc has been proposed with which it is possible to image optical data without the use of laser light, somewhat in the manner of microfilm or microfiche. Such a system is disclosed in British Patent Specification No. 1,091,981 and comprises the use of a disc having a spiral groove and between the turns of the groove a flat spiral land having a coating of photographic emulsion constituting a video track. A reading head is employed which has a pick-up which physically engages the groove for the purpose of accurately locating the reading head relative to the video track. Video data is recorded on the track so that a flying spot scanner can scan picture elements in a linear fashion.
For this purpose the reading head incorporates a light source and lens enabling an image of the video track to be passed along the recording head to a flying spot type pick-up tube. No method is provided for random-access as it is believed that the system of that specification is intended for sequential reproduction of images as in a video player.
An object of the present invention is to provide an optical data storage system which does not rely on laser light, but which is capable of efficient random access.